Small (micron) scale damage in polymeric materials is often difficult to detect, yet it compromises mechanical integrity and inevitably leads to failure. Strategies that enhance detection of damage are therefore important for improving safety and increasing reliability, while also reducing life cycle costs associated with regular maintenance and inspection. Moreover, systems that respond autonomously to self-report damage are appealing because no human intervention is required.
The development of self-reporting materials enables autonomous damage detection for improved safety and reliability of critical engineering components. For example, incorporation of mechanically sensitive molecules in polymeric materials through covalent or non-covalent modification facilitates color changes in response to macroscopic deformation.
Enhanced damage visibility in polymer composites has been achieved using a fluorescent dye contained within embedded hollow fibers. However, this method suffers from the absence of a “turn-on” mechanism, precluding its utility in transparent materials.
Fluorescence detection provides significantly enhanced sensitivity over absorption-based colorimetric methods. Conventional fluorophores are usually flat disc-like aromatic molecules with high planarity and rigidity which result in efficient light emission. However, typical fluorophores also exhibit diminished emission with increasing concentration. These molecules experience strong molecular interactions and thus suffer from a severe aggregation-caused quenching (ACQ) effect. Previous techniques to tackle the ACQ problem have focused on preventing aggregation but have resulted in limited success.
Visualization of damage has also been accomplished using microcapsules containing a conjugated monomer in combination with an embedded polymerization catalyst as well as pH-sensitive dyes that change color upon reaction with an auxiliary compound or with certain functional groups present in the polymer matrix. Chemical activation of an embedded fluorogenic molecule and formation of a charge-transfer complex using a dual capsule system has also been described.
Nevertheless, the foregoing current damage detection methods generally rely on chemical reactions to elicit a response and are highly material-dependent or complicated by multiple components. The solution presented in this disclosure provides a damage detection approach which does not rely on chemical reactions but instead on the unique feature of aggregation-induced emission (AIE) luminogens, which have been used for other applications in areas such as solid state optoelectronic devices and rewritable media for optical data storage.